In superheterodyne receivers, a relatively common problem is an undesired output, interfering with the desired outputs, which interference may occur when the receiver is tuned to a signal whose carrier frequency is very close to being an integral multiple of the receiver's intermediate frequency. The IEEE Standard Dictionary of Electrical and Electronics Terms [ANSI/IEEE STD 100-1984, Third Edition Aug. 10, 1984, published by the Institute of Electical and Electronics Engineers] on pp. 458-459 defines the phenomenon as follows: "Intermediate-frequency-harmonic interference (superheterodyne receivers). Interference due to radio-frequency-circuit acceptance of harmonics of an intermediate-frequency signal." In IEEE STD 185-1975 [(IEEE/IHF Standard), Standard Methods of Testing Frequency Modulation Broadcast Receivers] on p. 24, Section 6.22-Regeneration, it is noted that this type of interference is called "tweet", a term that has been widely used in the receiver industry for many years and is used herein. While the condition may occur in any type of superheterodyne receiver, it has been particularly troublesome in amplitude-modulation (AM) receivers designed for reception of signals in the AM broadcast band (535-1605 kHz). However, the invention is not limited to application in receivers of that type only, but is also applicable to superheterodyne receivers designed for other frequency bands, and for types of modulation other than AM. For purposes of clarity, receivers of sound intelligence, such as AM radio receivers, will be used as the basis of the ensuing discussion.
In the past, radio receivers have employed continuous tuning, whereby a receiver could be tuned uninterrupted across the frequency band, or have used push-button arrangements which enabled the listener to tune incrementally to preselected stations. In either case, if the receiver were tuned to a signal whose carrier frequency was very nearly an integral multiple of the receiver's intermediate frequency (IF), an annoying interfering tweet would occur if there were sufficient undesired regenerative coupling or feedback between the receiver's IF circuits and the antenna or RF circuits.
More recently, synthesized frequency generation has become increasingly popular for providing the local oscillator frequency required by a superheterodyne receiver's frequency converter, in which the received signal frequency is converted to an intermediate frequency.
Frequency synthesizers for superheterodyne receivers may employ a variety of design approaches, but generally have some features in common, such as: (1) a frequency-stable master oscillator operating at a first reference frequency from which the desired frequencies are derived, and (2) the capability to provide in incremental steps the local oscillator frequencies required by the superheterodyne receiver's frequency converter to receive signals broadcast on any assigned frequency in the frequency band for which the receiver was designed.
Frequency synthesizers generally provide a more precise frequency for the frequency converter than was provided by earlier tuning systems. In the special case where the received signal frequency is a harmonic of the IF, a frequency synthesizer theoretically could provide a synthesized frequency which would result in an IF which was an exact integral submultiple of the signal frequency, provided the signal carrier was also at its exact specified frequency. In this case, if there were the regenerative paths in the receiver required to produce tweet, the tweet, being at zero frequency, would not be heard. However, because of normal tolerances in receiver components, thermal and aging effects, and allowable tolerances in signal frequencies, the occurence of the zero-tweet condition just described is highly unlikely, and in normal operation tweet can be expected to be observable in the receiver's output.
It is, therefore, an object of the present invention to provide in a superheterodyne receiver equipped with synthesized-frequency tuning, means to detect frequency relations which may produce a tweet, and to automatically change the IF in a direction which prevents, reduces or eliminates the tweet.